1. Field of the Invention
The invention relates to soil remediation systems and processes and, more particularly, relates to a system and process for remediating highly contaminated soil by low temperature thermal desorption.
2. Discussion of the Related Art
Soils at a large number of both abandoned and operating industrial sites are contaminated with waste oil, fuel oil, and a variety of other combustible or even explosive organic substances. The demand for the remediation of such soils has increased at the same time that remediation has become increasingly problematic. More specifically, both public awareness of the dangers of on-site storage of such substances and government regulations prohibiting the continued storage of such substances has required the clean-up of sites that have stored large quantities of such substances for many years. This clean-up must extend in the case of each site beyond the substances themselves to the remediation of thousands of tons of soil contaminated with such substances. However, dwindling landfill space and increasingly strict government regulations have rendered more difficult the treatment of such soil. The two methods commonly employed for remediating soils are high-temperature incineration and low-temperature thermal desorption.
High-temperature incineration subjects materials to sufficiently high temperatures to alter the molecular structure of most of the materials subject to the process, thus forming a concentrated ash. This process, though capable of remediating even highly contaminated soils, suffers from several drawbacks and disadvantages. High-temperature incineration requires the consumption of large quantities of fuel and thus is relatively expensive. Moreover, by destroying all organic materials in the soils, incineration may render treated soils unsuitable for reuse in some applications.
Low-temperature thermal desorption (LTTD) does not alter the molecular structure of soils but instead removes contaminants from the soils by forced evaporation in a heated drum forming a primary treatment unit (PTU). The vaporized volatiles are then exhausted from the PTU and oxidized in a separate high temperature afterburner requiring much less fuel than an incineration system.
LTTD is often preferred to high-temperature incineration for a variety of reasons. Primary soil treatment by the PTU of an LTTD system occurs at relatively low temperatures of 500.degree.-800.degree. F. and thus requires dramatically less fuel than incineration systems. LTTD also does not require equipment to be modified to operate under extremely high temperatures required for incineration. Accordingly, LTTD systems are more economical than incineration units both in terms of capital investment and operating costs. Moreover, the treated soils are unchanged apart from having the organic materials destroyed and thus are suitable for return to the site, further reducing operating costs.
One problem with LTTD systems is that petroleum and similar contaminants are highly combustible or even explosive when in their vaporized state. Accordingly, when removed by vaporization in the PTU of an LTTD system, the levels of such contaminants must be maintained at levels well below their lower explosive limit (LEL). Contaminant concentrations above this limit could cause explosions in the ductwork leading to the afterburner. Thus, LTTD systems, though in most instances preferred over incineration systems for reasons described above, were heretofore thought unsuited for the treatment of highly contaminated soils which typically contain in excess of 3-4% by weight of hydrocarbons or other volatile organic substances. In fact, it was heretofore thought LTTD systems could not be used to remediate safely soils containing more than 1-2% by weight of such substances. Controlled treatment of soils having contaminant concentrations above this level was thought impossible because it was thought that such treatment could cause the gas stream fed to the STU to contain contaminant levels approaching or even rising above the LEL, thus creating the danger of an explosion before dilution air could be added to the gas stream. Accordingly, LTTD systems could heretofore handle highly contaminated soils only if such soils were diluted with clean soils prior to treatment to reduce the contaminant concentration levels to the maximum permissible operating range of 1-2%. This dilution requirement has heretofore rendered LTTD systems uneconomical for treatment of large quantities of highly contaminated soils.